Update abig.md

Add description and no molecular mechanism AbiG #195 (closed) .

requested review from @jcury

assigned to @ftesson

added DefenseSystem label

added 9 commits

• defb05ae...af4ef993 - 8 commits from branch main
• 79587d7d - Merge branch 'main' into 'ftesson_abig'

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added 65 commits

• 79587d7d...9fd810f9 - 63 commits from branch main
• 2c94dbcf - Merge branch 'main' into ftesson_abig
• e7783f88 - merge main correct misspelling

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merged

mentioned in commit 99a0d69f